1. Technical Field
The present invention relates to a shock absorber which includes a damping force adjusting apparatus which controls a flow of an internal working fluid generated according to extension and contraction motions and adjusts a damping force.
2. Related Art
For example, a shock absorber which is used as a front fork which suspends a front wheel of a motorcycle to a vehicle body, includes a cylinder in which an oil serving as a working fluid is enclosed, a piston which is slidably fitted into the cylinder, a piston rod which is connected to the piston and extends outside the cylinder, and a damping force adjusting apparatus which controls a flow of oil by sliding of the piston in the cylinder and adjusts the damping force.
In this kind of the hydraulic shock absorber, a damping force adjusting apparatus which largely changes damping force characteristics in a medium-high speed region of a piston speed is known, in which a pilot chamber is formed on a rear portion of a disc valve which selectively opens and closes an oil path provided in the piston, and the pilot chamber communicates with a cylinder chamber in an upstream side of the disc valve via a fixed orifice and communicates with a cylinder chamber in a downstream side of the disc valve via a variable orifice (for example, refer to Patent Literature 1 (JP-UM-A-S62-155242)). According to this kind of the damping force adjusting apparatus, a passage area between two chambers in the cylinder is adjusted by opening and closing the variable orifice, a pressure (back pressure) of the pilot chamber is changed by a pressure loss generated in the variable valve, and thus, it is possible to adjust a valve opening pressure of the disc valve. Accordingly, it is possible to widen an adjustment range of the damping force characteristics.
However, according to the above-mentioned damping force adjusting apparatus, when a flow path area of the variable orifice is decreased so that the damping force is adjusted to a hard side, as shown in FIG. 14, an opening degree of a disc valve 101 decreases as the pressure of the pilot chamber increases, and thus, a gap between the disc valve 101 and a valve seat 102 decreases. Accordingly, a speed of the working fluid passing through the gap is increased, and thus, the working fluid becomes a jet stream. Therefore, a pressure at the gap between the disc valve 101 and the valve seat 102 is decreased, a force in a valve closing direction acts on the disc valve 101, and the damping force is increased. As a result, particularly, there is a problem that an excessive damping force is generated in the medium-high speed region of the piston speed.
Accordingly, as shown in FIG. 15, Patent Literature 2 (JP-A-H10-246271) suggests a damping force adjusting apparatus which adopts a configuration in which an outer circumferential edge of a disc valve 201 is bent to a valve seat 202 side and a guide portion 201a is formed. According to the last mentioned damping force adjusting apparatus, a jet stream of a working fluid passing through a gap between the disc valve 201 and the valve seat 202 when the valve is opened is ejected at an ejection angle θ along an outer circumferential portion of the valve seat 202 toward a gap between the valve seat 202 and the guide portion 201a of the disc valve 201. Accordingly, the force in the valve closing direction which acts on the disc valve 201 is decreased, and particularly, it is possible to prevent an excessive damping force in the medium-high speed region of the piston speed from being generated.